This invention relates to a process for synthesizing complete mixed polyol esters, that is, polyol esters having at least two different ester groups and no hydroxyl groups. More particularly, this invention relates to a process for esterifying partial polyol esters with minimal rearrangement of ester groups either by intermolecular or intramolecular acyl group exchange, and to confectioner's hard butter compositions prepared in such fashion. The term "partial polyol ester" is used herein to denote a polyol which is partially, that is, incompletely, esterified and as a consequence contains at least one hydroxyl group.
The instant process provides mixed polyol esters with specific ester groups substantially all at specific polyol hydroxyl sites. This process is especially useful for providing confectioner's hard butter compositions from inexpensive raw materials such as lard and palm oil. Such compositions are useful as substitutes for cocoa butter in chocolate candies.
To be useful as a confectioner's butter, a triglyceride should optionally have the following characteristics: it should be brittle solid up to about 77.degree. F.; it should have a relatively narrow melting range; and it should be completely liquid at about 95.degree. F., i.e., slightly below body temperature. Such melting characteristics contribute glossy coatings, absence of stickiness and favorable volume changes during confectionery product molding. These unique melting characteristics make confectioner's butters such as cocoa butter desirable for use in confectionery products, especially chocolates. However, cocoa butter is relatively expensive and must be imported, even when domestic fats which can be used to produce acceptable confectioner's butters are in plentiful supply at much less than the cost of cocoa butter. For many years, therefore, attempts have been made to provide from readily available and cheaper fats a product that can be used to replace at least part of the cocoa butter in chocolates and other confectionery products that normally contain cocoa butter.
In this search for alternate confectioner's butters, it has been determined that the advantageous physical characteristics of such butters are derived from the arrangement of the fatty acid substituents in their glycerides. Analytical tests have shown that cocoa butter comprises principally fatty acid triglycerides wherein a major proportion of the oleoyl substituents on the glycerol molecule are in the 2-position, e.g., 1-palmitoyl-2-oleoyl-3-stearoyl glycerol, and minor amounts of triglycerides having a different order of substitution of the palmitoyl, oleoyl and stearoyl groups on the glycerol molecule. Accordingly, palmitoyl-oleoyl-stearoyl triglycerides having a major proportion of the oleoyl groups in the 2-position of the glycerol molecule would provide the desired confectioner's hard butter compositions were such triglycerides readily available.
With most esterification procedures, the synthesis of such position-specific triglycerides is impossible since substantial ester group rearrangement occurs during esterification of specific partial glycerides, the synthesis of which is known in the prior art. Thus, acylation of 1,3-diglycerides with oleic acid and a conventional acid esterification catalyst provides only a minor proportion of triglycerides having an oleoyl group at the 2-position, where this group must occur to provide the desired confectioner's butter.
Feuge, Willich and Guice, the Journal of the American Oil Chemists Society, July 1963, pp. 260-264, demonstrate that ester group rearrangement ordinarily occurs during the esterification of partial glycerides, and, at page 260, point out that hydrochloride, sulfuric and hydrocarbyl sulfonic acids, which are widely used as esterification catalysts, cause ester group rearrangement. Accordingly, these acid catalysts are not suitable for preparing the desired position-specific (i.e., 2-oleoyl) triglycerides for use as a confectioner's butter. Similarly, ester group rearrangement ordinarily occurs during esterification of partial polyol esters other than glycerides, e.g., during esterification of partial 1,2-propylene glycol esters.
One known method for synthesizing a synthetic confectioner's butter which is similar to cocoa butter comprises reacting a diglyceride having palmitolyl and stearoyl groups at the 1- and 3-positions with oleoyl chloride; see U.S. Pat. No. 3,012,890. Further, it is known in the prior art that, in general, acid chlorides can be used to specifically esterify mono- and diglycerides. The use of acid chlorides for specific esterifications has many undesirable aspects, however. For instance, acid chlorides are very corrosive and their use involves handling problems.
U.S. Pat. Nos. 3,410,881 and 3,337,596 disclose the use of perchloric acid as an effective catalyst for preparing a synthetic cocoa butter without rearrangement of the ester groups. However, perchloric acid is known to be explosive and its use in the presence of organic compounds is preferably avoided.
The copending application of Yetter, entitled "Process of Synthesizing Complete Mixed Polyol Esters," Ser. No. 242,139, filed Apr. 7, 1972, discloses the use of perfluoroalkanesulfonic acid catalysts in position-specific triglyceride syntheses. While effective for the intended use, such perfluorinated catalysts are quite expensive and are in relatively short supply.
The concurrently filed application of Yetter, entitled, "Process for Synthesizing Complete Mixed Polyol Esters," Ser. No. 279,577, filed Aug. 10, 1972, discloses the use of hydrogen bromide in position-specific triglyceride syntheses. While effective for the intended use, HBr is corrosive and can be difficult to handle.
It has now been found that boron trifluoride (and sources thereof) catalyzes the esterification of partial polyol esters without substantial ester group rearrangement. It is surprising that this particular acid catalyzes esterification reactions with only minimal ester group rearrangement, inasmuch as the previously noted catalysts are all protonic acids, rather than Lewis acids, as is BF.sub.3.
While boron trifluoride has been previously employed in esterification reactions (see, for example, U.S. Pat. Nos. 3,112,239; 3,242,200; 3,035,069; 3,182,034 and 2,414,999), it has not been heretofore suggested as a position-specific esterification catalyst. To the contrary, U.S. Pat. No. 2,634,279, teaches the use of boron trifluoride as an ester interchange catalyst. The general interchange of ester groups is, of course, the precise problem which must be avoided in the preparation of position-specific triglycerides in the manner of the instant invention. In light of the ability of BF.sub.3 to catalyze ester interchange, its use as a position-specific esterification catalyst in the manner of the instant invention is surprising.
It is an object of this invention to provide a process for synthesizing complete mixed polyol esters, especially triglycerides, with relatively little rearrangement of ester groups either by intermolecular or intramolecular exchange. It is a further object herein to provide a process for synthesizing specific complete mixed polyol esters without using perchloride acid, acid chlorides or perfluoroalkyl sulfonic acids or hydrogen bromide. Yet another object of this invention is to provide a process for the preparation of a confectioner's hard butter composition. These and other objects are obtained herein as will be see from the following disclosure.